The best way to download a dist, is by going to its website. However if you don't know what the websites are than you can use google to find them or visit http://www.linux.com/distributions/ for a list of distrobutions and its sites.

If you really want to learn how to use Linux http://slackware.com/ . Best distro by far in my experience except for the obvious it's package management system. Don't expect it to hold your hand along the way, there are no GUI tools included but in the end if you are successful with it you will get a rock solid OS that can be as fast as you make it be plus hopefully you will have a newfound understanding on the inside workings of Linux.